This invention generally relates to the art of electrical connectors and, particularly, to an electronic device wherein terminal pins are electrically coupled through holes in a flat flexible circuit or substrate.
A variety of electrical connectors are designed for utilization with a flat flexible circuit which may be mounted directly on the connector or connected in circuit with terminal pins on the connector. Generally, a flat flexible circuit includes a flat flexible dielectric substrate having one or more holes therein for receiving one or more terminal pins. A ductile conductive film or other circuit trace system is deposited on the substrate in an area at least about the hole or holes. The terminal pins are inserted into the holes in the substrate to establish electrical and mechanical connections between the pins and the flat flexible circuit. Normally, each hole is smaller in diameter than a respective pin. Alternatively, the pin may be punched through the flat flexible circuit to establish the electrical and mechanical connection therewith.
In order to assure good electrical and mechanical connections in these types of electronic devices or electrical connectors, solder or other adhesives often are used. For instance, in U.S. Pat. No. 4,970,624, dated Nov. 13, 1990 and assigned to the assignee of the present invention, uni-axial adhesive is deposited on the flat flexible circuit about the hole which is penetrated by the terminal pin. The adhesive includes a non-conductive base incorporating randomly spaced conductive particles. When the terminal pin is forced through the adhesive, a portion of the adhesive is carried with the terminal pin between the pin and the flat flexible circuit. The carried portion of the adhesive is compressed for establishing contact between the conductive particles and, thereby, conductivity between the terminal pin and the flat flexible circuit, leaving the adhesive other than that portion in a non-conductive state. Such adhesives often are called xe2x80x9cZ-axisxe2x80x9d adhesives. These adhesives were developed to replace soldering techniques which require specific temperature resistant components and substrates.
Conductive adhesives are used in other applications involving flat flexible circuits. For instance, in U.S. Pat. No. 5,456,616, dated Oct. 10, 1995 and assigned to the assignee of the present invention, the connector housing is fabricated of a die cast metallic material, such as of magnesium, aluminum or the like. The ductile film on the flat flexible circuit is fabricated of a different metallic material, such as copper or the like and, in fact, may be plated with still a different metallic material such as a tin/lead alloy. The conductive film on the flat flexible circuit acts as a ground plane against the rear face of the connector housing. The housing has a plurality of pins which project through holes in the flat flexible circuit. Using a xe2x80x9cZ-axisxe2x80x9d adhesive between the housing pins and the flat flexible circuit not only is expensive, as described above, but the conductive interface between the different metal components is limited to the areas of pressure. Consequently, that patent teaches the use of an omni-directional conductive adhesive deposited on the conductive film over the areas of the holes therein, the conductive adhesive expanding the conductive interface between the metal housing and the metal ground plane defined by the conductive film.
Although such uses of conductive adhesives, whether the adhesives are Z-axis adhesives or omni-directional adhesives, serve their intended purposes in certain applications, they are relatively expensive both in the cost of the adhesives as well as their methods of use. In addition, the use of either type of conductive adhesive is costly in terms of secondary operations and costs associated with the metal particles, not to mention the problem of clogging adhesive dispensers by the metallic particles.
Because of the problems associated with the use of conductive adhesives, a unique system was devised as disclosed in U.S. Pat. No. 5,384,435, dated Jan. 24, 1995 and assigned to the assignee of the present invention. That patent solves the problems with conductive adhesives by establishing an electrical connection directly between the terminal pin and the flat conductor on the flat flexible substrate by controlling various parameters between the pin and the substrate. In the system of the ""435 patent, the terminal pin is round in cross-section, and the round pin is inserted into a round hole in the flat flexible substrate. Although this system has proven quite effective, there continue to be a need for further improvements, and the present invention is directed to that end.
For instance, it has been found that a square cross-sectional pin can improve the column strength of the terminal pin to reduce the possibility of buckling of the pin in the mating direction (i.e. axially of the pin), without changing the pitch or spacing between the pins in a connector arrangement. In other words, if a round pin were to have a diameter equal to the cross-dimension between opposite diametral corners of a square pin, such large round pins would of necessity increase the pitch or spacing between adjacent pins and, therefore, increase the size of the overall connector. On the other hand, square pins can create problems by causing tearing of the hole in the substrate unless a particular dimensional relationship exists between the square pins and the round holes in the substrate. The present invention is directed to solving these problems or dilemmas.
An object, therefore, of the invention is to provide a new and improved system for mounting electrical terminals, such as terminal pins, in a flat flexible substrate or flexible circuit.
In the exemplary embodiment of the invention, an electronic device includes a flat flexible dielectric substrate or circuit which is less than 0.050 inch thick. The substrate has a generally round hole of a given diameter. Typically, a ductile conductive film is deposited on the substrate in an area at least about the hole. A substantially square terminal pin is inserted into the hole in the substrate. The pin has a given cross-dimension between opposite diametral corners thereof greater than the diameter of the hole. The difference between the cross-dimension of the pin and the diameter of the hole is on the order of 7% to 67% of the diameter of the hole. These parameters establish a good connection between the square pin and the conductive film on the flexible substrate without tearing the flexible substrate.
As disclosed herein, the hole in the substrate can be prepunched or drilled. The substrate may be fabricated of a material such a polyimide, although other materials as polyester and the like are appropriate. The ductile conductive film may be of copper or other conductive thick film material. It is contemplated that the terminal pin and the ductile conductive film both can be plated with a common plating material. In one embodiment of the invention, the plating material is a tin/lead alloy.
Other objects, features and advantages of the invention will be apparent from the following detailed description taken in connection with the accompanying drawings.